1. Field of the Invention
The present invention relates to a pedal device, especially to an accelerator pedal for vehicles, capable of applying a restoration force to a pedal in accordance with a depression distance of the pedal. More particularly, the present invention is concerned with an accelerator pedal device which can introduce a specific hysteresis to the relationship between the depression distance of an accelerator pedal and the corresponding force required to depress the pedal.
2. Description of Related Art
In general, an accelerator pedal device used in connection with an internal combustion engine employs a spring which is strained in accordance with the distance by which the accelerator pedal is depressed. The restoration force produced by this spring, which increases in proportion to the distance that the pedal is depressed, is transmitted back to the accelerator pedal through a mechanism including a control cable and other elements, so as to restore the pedal to its original rest position. Meanwhile, a throttle control device, referred to as "linkless throttle device", has been proposed in which the depression distance of the accelerator pedal is electrically detected. The throttle opening is controlled in accordance with the detected pedal depression distance of the pedal. In this linkless throttle device, the engine does not apply a load to the accelerator pedal, so the position of the accelerator pedal is determined only by the balance between the restoration force produced by the spring and the manual depression of the accelerator pedal. Consequently, any change in the manual force exerted on the accelerator pedal is directly related to a change in the depression distance of the accelerator pedal. This makes it extremely difficult to maintain a constant level of pedal operation, leading to deterioration in the engine controllability.
In order to solve the above-described problem, an accelerator pedal associated with the above-described linkless throttle device employs a suitable resistance device which poses resistance against the movement of the pedal. Such a resistance device introduces, as explained below, a certain hysteresis to the relationship between the degree of operation of the accelerator pedal (i.e., effectively, the pedal depression distance) and the manual force applied to the accelerator pedal, thereby contributing to improved driveability. It is assumed here that the force being applied to the accelerator pedal is slightly reduced after the pedal is stepped down to a certain position. In such a case, the restoration force applied to the accelerator pedal exceeds the manual force being applied to the pedal, but the pedal is not moved back immediately because of the resistance posed by the resistance device. Thus, the pedal does not move back until the manual force applied to the pedal is reduced to a level which is less than a value obtained by subtracting the movement resistance from the restoration force. It is thus possible to operate the pedal at a constant level of operation, regardless of any slight change in the manual force exerted on the accelerator pedal.
In general, the accelerator pedal operation is usually held constant during heavy load operation of the engine, which requires a large amount of accelerator operation. On the other hand, an operation of the engine with a small amount of accelerator pedal operation requires delicate and frequent adjustment of the accelerator pedal operation. Therefore, the hysteresis of the accelerator control system is preferably so determined to be comparatively large in the region where the accelerator pedal operation amount is large and to be comparatively small in the region where the accelerator pedal operation amount is comparatively small. A large hysteresis in the region of small accelerator pedal operation amount tends to cause a dangerous risk of a phenomenon called "returning failure" in which the accelerator operation cannot be reduced to zero when a depressing force is removed.
Accordingly, a device has been proposed in, for example, Japanese Patent Laid-Open Application No. 4-128519 in which the resistance to movement of an accelerator pedal increases in accordance with increase in the depression distance of the accelerator pedal. In this device, the hysteresis imparted to the relationship between the depression distance of the accelerator pedal and the force exerted on the accelerator pedal is increased in accordance with an increase in the pedal depression distance so as to maintain good driveability over the entire range of amount of accelerator pedal operation.
The known accelerator pedal device of the kind described is susceptible to a returning failure when the spring is at least partly damaged, thereby reducing the restoration force. For instance, current automobiles employ a spring system having two springs to ensure that the accelerator pedal can safely be returned to an inactive rest position even if one of the two springs is broken. In the accelerator pedal device employing this dual spring system, breakdown of one of the springs reduces the total restoration force to about half the restoration force which would be produced when both springs are available. This reduced restoration force is often smaller than the resistance to movement of the pedal, so that the accelerator pedal may fail to move back to its rest position where the throttle is closed, even when the pedal is relieved of a pedal depressing force.